The present invention generally relates to an image pickup tube employed in a television camera or the like, and more particularly to a structure of electrostatic deflecting electrodes for a magnetic focusing and electrostatic deflecting image pickup tube (hereinafter referred to as MS image pickup tube).
In the heretofore proposed MS image pickup tube, an electron beam produced therein is focused by a magnetic coil disposed around a tubular glass envelope or glass tube, while the electron beam is deflected by two pairs of electrostatic deflecting electrodes, i.e. horizontal and vertical deflecting electrodes, formed on the inner surface of the glass tube. FIG. 1 of the accompanying drawings shows a hitherto known MS image pickup tube in a sectional view. Disposed within a glass tube 1 at one end thereof is an electron gun 2 which is composed of a cathode 21, a first grid 22, a second grid 23 and a beam disc 24 having a limiting aperture formed therein. The electron gun 2 emits an electron beam 3. Disposed within the glass tube 1 at the other end are photoconductive target 4 scanned by the electron beam 3 and a mesh electrode 5. Formed on the inner surface of the glass tube 1 are electrostatic deflection electrodes 6 which generate deflection electric fields for causing the target 4 to be scanned by the electron beam 3 in both horizontal and vertical directions. A focusing coil 7 is disposed externally around the glass tube 1 to generate a focusing magnetic field for focusing the electron beam 3 directed onto the opposing surface of the target 4. A cylindrical electrode 8 is interposed between the mesh electrode 5 and the deflection electrodes 6. The mesh electrode 5 and the cylindrical electrode 8 have an equal potential applied thereto. An electrostatic lens is formed under the influence of potential difference between the cylindrical electrode 8 and the deflecting electrodes 6. This electrostatic lens is referred to as the collimating lens which is effective for eliminating radial landing error of the electron beam 6 deflected by the deflection electrodes 6. The mesh electrode 5 serves to generate a decelerating electric field between the target 4 and the mesh electrode 5 for the purpose of allowing the lowvelocity scanning by the electron beam.
The deflection electrodes 6 are formed through vacuum evaporation of an electrically conductive material on the inner surface of the glass tube 1 and by dividing the electrically conductive layer into four discrete electrodes, each of which has a zigzag pattern and is separated from one another by using a laser beam or the like. The deflection electrodes 6 thus formed are referred to as the pattern yoke. FIG. 2A is a developed view of the pattern yokes 6 as viewed interiorly of the glass tube. Such zig-zag configuration of the pattern yoke is disclosed in U.S. Pat. No. 2,830,228 to Schlesinger i.e., edge portions of the deflection electrodes have sinusoidal curve, and called the curved arrow pattern yoke. FIG. 2B shows the pattern yokes 6 as viewed from the electron gun 2 within the glass tube 1, wherein the thickness of the electrodes is omitted from illustration. Referring to FIG. 2A, a line B.sub.1 B.sub.2 interconnecting the upper apices M of a zigzag pattern yoke is in the form of a helix extending from one end to the other end of the pattern yoke, revolving about the longitudinal axis of the glass tube. Referring to FIG. 2B, the angle of rotation of the helix, i.e. the central angle &lt;B.sub.1 OB.sub.2 formed between the points B.sub.1 and B.sub.2 at which the helix intersects opposite ends of the pattern yoke, respectively, is referred to as twist angle which will be represented by .omega.. In the case of the illustrated pattern yoke array, the twist angle .omega. is 180.degree.. In FIG. 2A, the twist angle is taken along the ordinate with reference to a line A.sub.1 -A.sub.2. The pattern yoke twisted at the twist angle 90 degrees is disclosed in U.S. Pat. No. 3,666,985 to Schlesinger. The pitch of the zigzag pattern yoke from one upper apex to the succeeding one is represented by L, and the number of pitch repetition is represented by n. Then, the length of the pattern yoke is given by nL. Of the pattern yokes, electrodes denoted by H.sup.+ and H.sup.- are, horizontal deflection electrodes to which voltages +V.sub.H /2 and -V.sub.H /2 superposed on bias voltage E.sub.C3, respectively, are applied for forming a horizontal deflecting electric field. The electrodes denoted by V.sup.+ and V.sup.- are vertical deflection electrodes to which voltages +V.sub.V /2 and -V.sub.V /2 superposed on bias voltage E.sub.C3, respectively, are applied for generating a vertical deflecting electric field.
In connection with the MS image pickup tube of the structure described above, it is taught in U.S. Pat. application Ser. No. 668,844 (field on Nov. 6, 1984) that the twist angle of the pattern yoke should preferably be about 30.degree. in order to enhance remarkably the uniformity of resolution.
Further, in the MS image pickup tube of the aforementioned type, the voltage applied to the mesh electrode can be correspondingly increased by increasing the twist angle. Increasing in the voltage of the mesh electrode in turn reduces the beam bending. With the phrase "beam bending", such a phenomenon is to be understood in which the path of electron is bent toward a bright portion of the target 4 projected with an optical image and which brings about raster distortion near a bright portion of the target as well as degradation of the resolution.
Accordingly, in order to enhance the uniformity of resolution and reduce the beam bending, it is desirable that the twisted pattern yokes be employed.
In the foregoing, structure and operation of the hitherto known MS image pickup tube has been described. Next, problems which the MS image pickup tube of the structure described above will be mentioned below.
FIG. 3 of the accompanying drawings is a developed view of a portion of the pattern yokes in the zigzag array which corresponds to two pitches. It will be seen that gaps .DELTA. are formed between the adjacent electrodes in the circumferential direction for the purpose of interelectrode insulation.
In the curved arrow pattern yoke illustrated in FIG. 3, in which the segments of the zigzag shaped deflection electrodes have sinusoidal shaped curves, the electrodes are configured such that a very sharp angle is formed at every upper apex of the zigzag profile and that the space between the adjacent deflection electrodes is much narrowed in the vicinity of the upper apex M when compared with the space at a location B. As a consequence, intensity of the electric field is increased in the vicinity of the upper apex, involving the danger that electric discharge may result in partially damaging damage partially or even eventually breaking some deflection electrodes. This in turn means that the uniformity of the deflecting electric field is injured with the raster distortion becoming correspondingly greater, resulting in lowering of the production yield of the image pickup tube.